The present invention relates to a process for bonding particulate material such as chips, wafers, strands, fibers, shavings or dust of various, optionally pre-dried, raw materials, such as size-reduced industrial waste and, in particular, materials containing lignocellulose, such as wood or annual plants, by spraying a bonding agent on the material which has been broken down into a layer of particles. The present invention also relates to an apparatus for carrying out the process, based on a known pneumatic apparatus for transporting material, comprising conveyor blowers, feed screw, pipelines, separators, compensating vessel and knife gate.
A process of the type indicated above has been disclosed in German Offenlegungsschrift No. 1,653,223, wherein a device designed to achieve fine sub-division is provided to produce a film of particles. The device is designed for processing fibrous material, such as wood fibers or fine wood dust in the form of short, fine, thin fibers. The apparatus for fine sub-division of the fibrous material consists essentially of a stationary screen cage or perforated wall and a stirrer arranged inside the cage. Connected to this device upstream thereof is a dosing device for the fibrous material. The stirrer in the device for fine sub-division is driven and has stirrer arms which push the fibrous material radially outwards through the perforations of the cage in superposed planes so that a film of particles is formed without the particles making contact with a wall and this film descends under free fall in the form of a hollow cylinder. This film of particles is bonded in two stages, once from the inside and once or more from the outside, by successively spraying bonding agent on to the particles of the film. The film of particles has a considerable thickness so that the jets of bonding agent sprayed from the inside to the outside of the film wet the film, to varying degrees and penetrate the film to varying depths. Application of the bonding agent from the inside is carried out without pressure, but application from the outside may be carried out under pressure. To counteract uneven application of bonding agent, agitator mixers are connected downstream of the apparatus. Alternatively, two wetting installations are provided in succession and the particles are passed through the two installations in succession. The investment in machinery for these specialized treatments is considerable.
For achieving more uniform bonding of the particles of a particle film, German Offenlegungsschrift No. 2,913,081 describes a process which may also be used for evenly applying bonding agent to flat wood chips of the type which are present in a wood chip board. This object is achieved by turning the film of wood chips between two stages of application of bonding agent, the bonding agent being applied predominantly to the front surface of the flat chips in the first stage and predominantly to the back of the flat chips in the second stage. A very thin film of particles is used in this process and again a considerable outlay in machinery is required.
German Offenlegungsschrift No. 2,653,683 discloses a process and an apparatus by which wood chips are formed into a ring which moves in a spiral while the chips are treated with bonding agent. The apparatus contains a high speed mixer shaft which produces the spirally moving ring of chips along the internal wall of the container. Centrifugal tubes dip with the outlet openings thereof for bonding agent into this ring of chips. These centrifugal tubes for the supply of bonding agent are distributed over the circumference and axis of the mixer shaft over the whole bonding and mixing zone. These centrifugal tubes serve not only to apply bonding agent to the chips, but also to exert a certain mixing action on the wood chips of the ring. The bonding agent leaves the ends of the tubes in the ring of chips in the form of droplets which are swept over the chips as the chips move past them. The bonding agent becomes uniformly distributed over the chips by the friction of the wood chips and the use of constantly changing relative velocities. In spite of the considerable length of the apparatus, homogeneous distribution is in fact not achieved, partly because the bonding agent in the form of droplets is transmitted in a highly concentrated form to individual chips.
A very similar apparatus is disclosed in German Auslegeschrift No. 2,304,262, but in this case the bonding agent is delivered through tubes in the wall of the mixer without application of pressure. The openings of the tubes are situated in the ring of chips. Here again, the chips remove the bonding agent in the form of droplets from the free end of each delivery tube and the bonding agent is transmitted to other wood chips by contact between chips and a wiping action.
These known types of apparatus also require the provision and operation of separate installations which entail additional expenditure.
Pneumatic chip transport devices comprising conveyor blowers, feed screws, pipelines, separators, compensating vessels and knife gates or the like are known, such devices serve to transport the chips, for example, from the place where they are produced to the place where they are to be processed. The known installations convey the chips in a loosened form together with a substantial proportion of transport air through the pipelines. The apparatus must be so designed that long transport paths may be traversed without the chips undergoing further size reduction or parts of the installation undergoing significant wear and tear.
Typical of such pneumatic conveyors are those described in the Chemical Engineers' Handbook, Perry and Chilton, Fifth Edition, 1973, pages 7-16 to 7-21 and 20-55 to 20-58.